U.S. Pat. No. 7,575,789 discloses a lining adhered to the interior surface of an oil well pipe, also known as oil production tube or down-hole oil pipe, the lining including a primer layer and an overcoat layer on the primer layer, the overcoat providing a non-stick perfluoropolymer surface. In a preferred embodiment, the primer layer is formed by spraying a liquid-based primer composition onto the interior surface of the pipe and drying or baking the primer composition to form the primer layer, followed by spraying a liquid composition onto the primer layer and drying and baking this composition to form the overcoat layer. This lining has performed admirably in oil well application, preventing the plugging of the oil well pipe by deposition of asphaltenes, paraffin wax, and inorganic scale present in the oil flowing through the pipe
A water-miscible organic liquid with a high boiling temperature, notably glycerin, which boils at 291° C., is included in the overcoat composition of U.S. Pat. No. 7,575,789 in a substantial amount to prevent the dried coating, prior to baking, from sloughing off of the primer layer within the interior of the pipe wherein the cylindrical surface is mostly non-horizontal. U.S. Pat. No. 5,502,097 discloses the use of such high boiling liquid in the coating composition to prevent sloughing off of the coating from a vertical surface. The interior of oil well pipe presents a continuum of surface varying from the non-horizontal, including vertical and overhanging (upper), looking down on the lower surface of the pipe. This sloughing off of the dried coating from non-horizontal surface is a problem, especially if the pipe being lined is moved or impacted during drying, until the baking of the dried layer fuses the perfluoropolymer present in the layer, resulting in the overcoat adhering to the primer layer. While the high boiling organic liquid is effective in resisting sloughing, its boiling temperature is less than that of the baking temperature, whereby the liquid volatilizes, often resulting in excessive smoking (fuming) of the layer. There is a need for solving the sloughing problem without having to volatilize so much organic liquid.
While, the pipe lining of U.S. Pat. No. 7,575,789 performs admirably well in oil well operation, there are occasions in oil recovery or maintenance of the oil well pipe that the interior of the pipe is exposed to pressurization and rapid decompression. U.S. Patent Publication 2009/0078328 discloses an Autoclave Test simulating the effect pressurization and rapid decompression on a pipe having a non-stick lining. The autoclave contains test fluids, liquid and gas, comparable to those encountered in an oil well and a non-stick lining that would be used for oil well pipe. The lining is formed from a primer layer on the metal surface, which could be that of the interior of the pipe, and an inner layer adhered to the primer layer and an outer layer adhered to the inner layer. The combination of the inner layer and outer layer is the overcoat for the primer layer. Pressurization forces test fluids to permeate into the coating, and rapid decompression results in the rapid exit of these penetrated fluids from the coating, causing blistering (layer separation) within the coating, namely between the primer layer and the inner layer of the overcoat. This blistering represents locations for potential corrosive attack and coating failure in actual service, wherein the coating forms the interior surface of oil well pipe. There is a need to increase the adhesion between the primer layer and overcoat of the pipe lining so that the lining has greater resistance to blistering should the occasion of rapid decompression of the pressurized interior of the lined pipe arise in actual down-hole service.
The outer layer of the oil pipe lining of U.S. Pat. No. 7,575,789 may contain particles that form a mechanical barrier against the permeation of water, solvents, and gases. While these particles are effective in protecting the metal surface of the pipe underlying the non-stick coating from corrosion, the coating is nevertheless susceptible to blistering upon rapid decompression. Apparently enough fluid permeates into the lining to give this disadvantageous result. While the presence of the permeation barrier particles, usually platelet in shape, protects the underlying pipe surface from corrosion, the inner layer of the overcoat containing these particles, exhibit decreased cohesive strength. Cohesive strength is the strength of the layer within itself, i.e. the ability if the layer to retain its integrity under stress, in contrast to the strength of the bond between the inner layer and adjacent layers. As a result of this reduced cohesive strength, the lining is subject to delamination from physical abuse that may be encountered in down-hole operation, for example being struck by a down-hole tool inserted into the pipe. This delamination has been found to be a failure within the thickness of the overcoat, i.e. within the inner layer of the lining of U.S. Pat. No. 7,575,789, by separation of the fluoropolymer constituent of the inner layer from the surface of the particles lying within the inner layer. The location of this delamination then becomes a point for corrosive attack on the underlying pipe surface and a loss of non-stick character inviting the buildup and plugging from one or more of asphaltenes, wax and inorganic scale. There is a need to increase the cohesive strength of the layer containing the barrier particles.